What's the difference between a hash and hash reference in Perl?

Question

I would like to properly understand hashes in Perl. I've had to use Perl intermittently for quite some time and mostly whenever I need to do it, it's mostly related to text processing.

And everytime, I have to deal with hashes, it gets messed up. I find the syntax very cryptic for hashes

A good explanation of hashes and hash references, their differences, when they are required etc. would be much appreciated.

Answer 1

A simple hash is close to an array. Their initializations even look similar. First the array:

@last_name = (   "Ward",   "Cleaver",   "Fred",   "Flintstone",   "Archie", "Bunker" ); 

Now let's represent the same information with a hash (aka associative array):

%last_name = (   "Ward",   "Cleaver",   "Fred",   "Flintstone",   "Archie", "Bunker" ); 

Although they have the same name, the array @last_name and the hash %last_name are completely independent.

With the array, if we want to know Archie's last name, we have to perform a linear search:

my $lname; for (my $i = 0; $i < @last_name; $i += 2) {   $lname = $last_name[$i+1] if $last_name[$i] eq "Archie"; } print "Archie $lname\n"; 

With the hash, it's much more direct syntactically:

print "Archie $last_name{Archie}\n"; 

Say we want to represent information with only slightly richer structure:

• Cleaver (last name)
  • Ward (first name)
  • June (spouse's first name)
• Flintstone
  • Fred
  • Wilma
• Bunker
  • Archie
  • Edith

Before references came along, flat key-value hashes were about the best we could do, but references allow

my %personal_info = (     "Cleaver", {         "FIRST",  "Ward",         "SPOUSE", "June",     },     "Flintstone", {         "FIRST",  "Fred",         "SPOUSE", "Wilma",     },     "Bunker", {         "FIRST",  "Archie",         "SPOUSE", "Edith",     }, ); 

Internally, the keys and values of %personal_info are all scalars, but the values are a special kind of scalar: hash references, created with {}. The references allow us to simulate "multi-dimensional" hashes. For example, we can get to Wilma via

$personal_info{Flintstone}->{SPOUSE} 

Note that Perl allows us to omit arrows between subscripts, so the above is equivalent to

$personal_info{Flintstone}{SPOUSE} 

That's a lot of typing if you want to know more about Fred, so you might grab a reference as sort of a cursor:

$fred = $personal_info{Flintstone}; print "Fred's wife is $fred->{SPOUSE}\n"; 

Because $fred in the snippet above is a hashref, the arrow is necessary. If you leave it out but wisely enabled use strict to help you catch these sorts of errors, the compiler will complain:

Global symbol "%fred" requires explicit package name at ... 

Perl references are similar to pointers in C and C++, but they can never be null. Pointers in C and C++ require dereferencing and so do references in Perl.

C and C++ function parameters have pass-by-value semantics: they're just copies, so modifications don't get back to the caller. If you want to see the changes, you have to pass a pointer. You can get this effect with references in Perl:

sub add_barney {     my($personal_info) = @_;      $personal_info->{Rubble} = {         FIRST  => "Barney",         SPOUSE => "Betty",     }; }  add_barney \%personal_info; 

Without the backslash, add_barney would have gotten a copy that's thrown away as soon as the sub returns.

Note also the use of the "fat comma" (=>) above. It autoquotes the string on its left and makes hash initializations less syntactically noisy.

Answer 2

The following demonstrates how you can use a hash and a hash reference:

my %hash = (     toy    => 'aeroplane',     colour => 'blue', ); print "I have an ", $hash{toy}, " which is coloured ", $hash{colour}, "\n";  my $hashref = \%hash; print "I have an ", $hashref->{toy}, " which is coloured ", $hashref->{colour}, "\n"; 

Also see perldoc perldsc.